Errors in optically transmitted data may be due to different causes, including distortion of the signal in the transmission medium. In free-space optical communications systems that propagate light through air, turbulence can be a significant source of channel impairment. For example, non-uniform refraction of the optical beam can be caused by small-scale fluctuations in air density that result from temperature or pressure gradients along the path of the optical beam. These atmospheric fluctuations can cause frequency-nonselective fades in the optical beam's power. The fade process has a correlation time which is typically much longer than the duration of a typical symbol in the optical beam, therefore negatively impacting the signal to noise ratio of the data stream.
To reduce the beam fading, some conventional technologies use adaptive optics. The adaptive optics correct the deformations of an incoming wavefront of light by measuring the distortions in the wavefront of the optical beam (e.g., using array with hundreds or thousands of detectors) and by compensating these distortions with adaptive optical elements, e.g., deformable mirrors. Typically, hundreds of actuators are needed to appropriately deform a mirror. However, the deformable mirrors and other active optical elements generally have relatively slow response time, therefore not being capable of correcting the fast-changing distortions in the wavefront of the optical beam. Furthermore, the active optical elements can be expensive.
Some other conventional technologies use an optical beam with relatively wide aperture to average-out the intensity variations across the beam, thereby reducing the fading. Generally, the required aperture of a beam that would overcome turbulence-induced fading becomes very large. However, for many practical applications of optical data transmission, the size of the aperture is limited, e.g., due to energy required to generate the beam or the allowable spacing among the beams. Furthermore, aperture-averaging does not correct for phase variations across the aperture, and is thus unsuitable for systems that utilize phase-sensitive signaling or single-spatial mode coupling. Accordingly, there remains a need for optical communication systems that can reliably transfer data through the atmosphere, where the atmosphere causes distortions in the wavefront and/or frequency-nonselective fades in optical power.